The ability to name your own price might sound like a deal too good to be true. Which is why take up of the idea has been a bit slow. That could change with the sudden arrival of Cardelmar.com, a German car hire broker that launched in the UK last week. Like most brokers, it displays lots of transparent prices from major car rental companies, but it also invites you to say how much you'd like to pay. Of course, this doesn't mean that you can suddenly find yourself the wheel of an Aston Martin for £1 a week, but it does mean you can take a lot of the tedium out of searching for car hire by outsourcing the hard work to someone else. You park up, quote a reasonable price and wait for Cardelmar to return with a suitable deal. It's a bit like an online version of haggling.

The trend for these "reverse auctions" was started by priceline.co.uk, an accommodation site selling unoccupied hotel rooms at the last minute. Priceline works like this: you select how much you're willing to pay for accommodation in a specific area, at a given time and within a certain quality threshold. Priceline then scours all its available deals and if there's something within your particular margin, it selects a room for you. This is also known as an "opaque" booking system, meaning that you don't actually receive details of your precise hotel until after you've paid.

You can also do something similar with flights. The US site cFares.com allows you to scour hundreds of different airlines and compare their prices. But its chief innovation is that you can set it to continue searching while you are away from your computer, even for days at a time, so if you are searching for a flight and the price you find is not as low as you'd like, you can set a "virtual agent" to continue searching until a better one comes along - effectively naming your own price. The only real snag - apart from paying in US dollars - is that you have to give over your credit card details to reserve a price if it comes up in your absence, although you pay for nothing you don't approve.

So why isn't everybody naming their own price? Part of the reason is unfamiliarity with the concept, but the biggest reason may be psychological. Like the "winners curse" in a conventional auction, naming your own price can leave you with the feeling that you could've won the very same deal by naming a price that was even lower.